Growing up in Scotland, BBC editor Martin Davidson remembers his mother's German father, Bruno Langbehn, as the perfect grandfather, full of jokes and entertaining stories. But after the old man died in 1992, Davidson discovered that Langbehn, in many ways, had also been the perfect Nazi: among the first to join the Nazi party (six years before Hitler came to power), assigned to the elite SD security police, and a member of the "Werewolf" resistance that waged guerrilla warfare against the victorious Allies at war's end.